The present invention relates to the plate-like magnetoplumbite type ferrite particles for magnetic recording comprising the Ba-containing plate-like magnetoplumbite type ferrite particles having the composition of AO.n{(Fe.sub.1-X M.sub.X).sub.2 O.sub.3 }, wherein A represents Ba or Ba and Sr, M represents Zn and Ti, or Zn, Co and Ti, n=6.5-11.0, and x=0.05-0.25 and the change of coercive force of the particles at a temperature of 20.degree.-120.degree. C. in the range of -2.0 Oe/.degree. C. to +2.0 Oe/.degree. C., and magnetic recording media made by coating on a substrate a resin composition containing the Ba-containing plate-like magnetoplumbite type ferrite particles having the composition of AO.n{(Fe.sub.1-X M.sub.X).sub.2 O.sub.3 }, wherein A represents Ba or Ba and Sr, M represents Zn and Ti, or Zn, Co and Ti, n=6.5-11.0, and x=0.05-0.25, the change of coercive force of the magnetic recording media at a temperature of 20.degree.-90.degree. C. being in the range of - 2.0 Oe/.degree. C. to +2.0 Oe/.degree. C.
With the advancement of technology for longer recording time, and smaller size and weight of the magnetic video and audio recording and reproducing devices in recent years, the necessity has grown for the higher performance of the magnetic recording media such as magnetic tapes, magnetic discs, etc. Especially, high-density recording by magnetic recording media is required acutely.
Magnetic recording media are generally produced by coating a resin composition containing magnetic particles on a substrate.
According to the conventional recording system, the acicular magnetic particles such as acicular .gamma.-Fe.sub.2 O.sub.3 particles or Co-coated acicular .gamma.-Fe.sub.2 O.sub.3 particles have been oriented in the longitudinal direction on the plane of the magnetic recording medium and recording has been made by making use of the remanent magnetization in the direction. For attaining high-density recording, it is necessary to shorten the recording wavelength. However, according to the conventional recording system, because of the necessity of shortening the major axis of the acicular magnetic particles for short wavelength recording, the influence of the dimagnetizing field between the particles would increase to cause a drop of reproducing output, so that this system was disadvantageous for high-density recording.
As a high-density recording system, there has been proposed recently a so-called perpendicular magnetic recording system in which recording is performed by making use of the remanent magnetization in the direction perpendicular to the longitude of the magnetic recording medium, and this system is reaching the stage of practical application.
According to this perpendicular magnetic recording system, since the magnetic particles are oriented in the direction perpendicular to the longitude of magnetic recording medium and arranged so that the particles of opposite magnetic polarities adjoin each other, there hardly arises the influence of diamagnetizing field between the particles even when short wavelength recording is conducted. Further, since the adjoining particles strengthen their magnetization each other, the drop of output is lessened. Thus, this system is suited for high-density recording.
As mentioned above, the development of perpendicular magnetic recording media capable of high-density recording is most acutely required at present. As such magnetic recording media, there are known ones made by coating on a substrate a resin composition containing the Ba-containing plate-like magnetoplumbite type ferrite particles as for instance disclosed in Japanese Patent Application Laid Open (Kokai) No. 55-86103(1980).
Extensive studies have been conducted in recent years for the development of plate-like magnetoplumbite type ferrite particles and practical use of the recording media coated with such particles, and in accordance with the advancement of such studies, request has mounted not only for the improvement of electro-magnetic performance and durability which are normally required for the recording media but also for the magnetic (especially coercive force) stability to temperature (hereinafter referred to simply as temperature stability).
Such request is based on the following fact. Magnetic recording is made by magnetizing the magnetic recording medium with a magnetic field generated by the magnetic head, but it becomes impossible to carry out the best mode of recording if the coercive force of the magnetic recording medium varies due to a rise of temperature caused by an invironmental change, even if the magnetic field of the magnetic head and the value of coercive force, which is a magnetization component of the recording media, have been adjusted at room temperature so as to allow the best mode of recording.
The properties such as electromagnetic performance of magnetic recording media are closely associated with the properties of the magnetic particles used therefor. Regarding the properties of the Ba-containing magnetoplumbite type ferrite particles, it is usually required of such particles that they have an average particle diameter of 0.01 to 0.3 .mu.m and are excellent in dispersibility, that their coercive force can be controlled in the range of 300 to 2,000 Oe, and that the magnetization value is as high as possible.
Control of coercive force in the range of 300 to 2,000 Oe is necessary for enabling recording by the currently used ferrite type magnetic heads. For reducing the coercive force of plate-like magnetoplumbite type ferrite particles, it has been proposed to replace part of Fe(III) in the ferrite with Ti(IV) and Co(II), or Co(II) and divalent metal ions M(II) such as Mn, Zn, etc.
As to the magnetization value, it is required that such magnetization value is as large as possible. This is mentioned in, for instance, Japanese Patent Application Laid Open (Kokai) No. 56-149328 (1981) which states: "Magnetoplumbite ferrite used as a material for magnetic recording media is required to have as large a degree of saturation magnetization as possible . . . "
The requirement that the plate-like magnetoplumbite type ferrite particles for magnetic recording should be as small in size as possible, especially not greater than 0.3 .mu.m, is also mentioned in many literatures. For instance, Japanese Patent Application Laid Open (Kokai) No. 56-125219 (1981) states: "The utility of perpendicular magnetic recording for longitudinal magnetic recording becomes apparent in the recording wavelength region below 1 .mu.m. For conducting satisfactory recording and reproduction in this wavelength region, it is desirable that the crystal particle size of the ferrite particle is less than about 0.3 .mu.m. However, since no desired ferromagnetism is provided when the crystal particle size becomes smaller than 0.01 .mu.m, a crystal particle size in the range of 0.01 to 0.3 .mu.m is practically required."
There is no end to the requirement for the improvement of properties of plate-like magnetoplumbite type ferrite particles for magnetic recording. The strong request for temperature stability in addition to the proper coercive force, large magnetization value and a proper average particle size is explained by the fact that while magnetic recording is made by magnetizing the magnetic particles in the magnetic recording medium with a magnetic field generated by the magnetic head, it becomes impossible to carry out the best mode of recording if the coercive force of the magnetic particels varies due to a rise of temperature caused by an environmental change, even if the magnetic field of the magnetic head and the value of coercive force, which is a magnetization component of the magnetic particles, have been adjusted at room temperature so as to allow the best mode of recording.
The Ba-containing plate-like magnetoplumbite type ferrite particels having a proper coercive force, a large magnetization, a proper average particle size and excellent temperature stability, and the magnetic recording media capable of high-density recording are most strongly required at present.
The plate-like magnetoplumbite type ferrite particles and the media coated with such particles are known as the particles and media that satisfy the property requirements, but the Ba-containing plate-like magnetoplumbite type ferrite particles tend to increase in coercive force as the temperature rises, and thus such particles are poor in temperature stability.
This phenomenon is evident from the disclosures in the June, 1986, issue of "METALS" (published by Agness) which states on page 10: "The coercive force of barium ferrite particles increases with the rise of temperature . . . ", and on page 12: "The coercive force of magnetic recording media and the change of saturation magnetization by temperature are decided by the magnetic particles used. It has been confirmed that even in the barium ferrite media, the coercive force and saturation magnetization show the same characteristics as the temperature characteristics of magnetic particles shown in FIG. 4."
As means for improving the temperature stability of Ba-containing plate-like magnetoplumbite type ferrite particels, there is known, for instance, the method disclosed in Japanese Patent Application Laid Open (Kokai) No. 61-152003(1986).
According to the method of this Japanese Patent Application (Kokai) No. 61-152003 (1986), the plate-like barium ferrite particles containing the elements for reducing coercive force, such as Co(II) and Ti(IV), are heat-treated in a reducing atmosphere at 300.degree.-700.degree. C., but this method has the problem that the heat-treatment increases the coercive force more than twice that before the treatment, and it is difficult to control the coercive force to a proper level. Therefore, when a magnetic recording media is produced by using the Ba-containing plate-like magnetoplumbite type ferrite particles, the obtained magnetic recording media has an elevated coercive force, and as a result it is difficult to perform recording with a conventional ferrite-made magnetic head.
Thus, the establishment of a process for producing the Ba-containing plate-like magnetoplumbite type ferrite particles having a proper level of coercive force and excellent temperature stability has been strongly desired.
As a result of extensive studies for obtaining the Ba-containing plate-like magnetoplumbite type ferrite particles having a proper level of coercive force and excellent temperature stability, it has been found that the Ba-containing plate-like magnetoplumbite type ferrite particles having the composition of AO.n{(Fe.sub.1-X M.sub.X).sub.2 O.sub.3 } (wherein A represents Ba or Ba and Sr, M represents Zn and Ti, or Zn, Co and Ti, n=6.5-11.0, and x=0.05-0.25) and the change of coercive force at a temperature of 20.degree.-120.degree. C. in the range of -2.0 Oe/.degree. C. to +2.0 Oe/.degree. C., obtained by mixing Ba material, Fe material and substituting elements, viz. Zn material, Ti material and Co material, in the specified ratios, heating and calcining the mixture at a temperature of from 750.degree. to 950.degree. C. in the presence of a flux, crushing the calcined product and washing it with water and an acid, are the plate-like magnetoplumbite type ferrite particles having a proper level of coercive and excellent temperature stability, and magnetic recording media coated with such particles also have a proper level of coercive force and excellent temperature stability. The present invention has been attained on the basis of this finding.